In the prior art, it is general than an instrument such as a square or a protractor is used for measuring a bending angle of a work sheet bent.
However, this method takes much time and causes an error for measuring due to a person who measures the angle. Further, this method prevents automation of the examining process notwithstanding a current of factory automation. Furthermore, this method prevents automatic determination of the final relative position of upper and lower dies thereby a desired bending angle is finally obtained after the present bending angle is detected during the bending process.
Such an automated bending processing is indicated in Japanese Patent 63-2687 (B) titled "A device for detecting bending angles of a work for a press brake", where a surface near the end of the work sheet is picked up by a visual sensor during bending processing, and the least bending angle of the work sheet is detected.
In the above patent, however, the bending angle is not obtained with high precision, because the sensor picks up the surface near the end part of the work sheet, at which generally a burr, warp, or taper exists. These obstacles make it difficult to detect an actual bending angle.
In particular, when a reflected light pattern is picked up at such an end part of the work sheet and when the end part is tapered, the image obtained from the reflected light is different from the actual light pattern since the reflected light has blurs. When a pickup device for light of permeability is used, a floodlight is required. In this case, the arrangements become bigger and therefore they are not used usually.
Furthermore, if the angle is detected at the end part, the composition of the frames of the bending machine would be limited since the position of the visual sensor is limited to the vicinity of the end part.
In addition, when the angle is detected at the end part, the angle detected is not representative one because the work is curved like a bow due to what is called "center flexure phenomenon."